1. Field of the Invention
The invention relates to motion transducers, and more particulary to transducers for measuring displacement and very small amplitude high frequency vibrations in hostile environmental conditions such as grease, oil, metallic sludge, corrosion, high ambient vibration, high temperature, electrical, and electromagnetic interference.
2. Related Art
Fiber optic devices for the detection and measurement of displacement and vibration have been disclosed by U.S. Pat. Nos. 3,273,447 to Frank and by 3,327,584 to Kissinger. Those devices have the capability to provide displacement masurements over a wide frequency range, including the range 0-10,000 Hz. However, the output of those devices attributable to Kissinger, which have been commerically marketed, are proportional to target surface motion as well as target surface reflectivity. To sense and measure motion precisely with these devices it is necessary to ensure that the target surface reflectivity is constant while measurements are being taken.
It has been found that accurate dynamic measurements can not be made with unencapsulated fiber optic devices in environments where there is contamination of the target surface or of the optical path to the target surface. Other non-contact motion transducers, such as eddy current or capacitive types can also provide high frequency displacement measurements, but they too suffer a degradation of performance when used in an environment that causes a metallic-based or any other electrically conductive contaminant to collect at the sensing tip. For example, when using any non-contacting devices to monitor bearing vibration in the manner disclosed in U.S. Pat. No. 4,196,629 to Philips (which is hereby incorporated by reference), it was found that bearings corrode in their housings and that the bearing lubricant can migrate into the sensing area, mixing with the corrosion debris as it migrates. The mixing of corrosion products and lubricant creates a metallic-based sludge that degrades the performance of any transducer that is sensitive to metallic substances or is dependent upon a clear optical path to the target.
Contact probes generally overcome fouling problems however these devices have a very limited frequency capability. Dial indicators and linear variable differential transformers are two examples of contact sensors that provide accurate position measurements but can not be used to measure vibrations in the displacement domain up to 10,000 hertz.
Miserentino et al., U.S. Pat. No. 4,171,645, disclosed displacement probes that combined non-contact fiber optic transducers with self-contained contact targets. Miserentino does not provide for high frequency measurement capability in any of his several embodiments. In fact, it is obvious from his embodiments that only low frequency vibration or simple position measurements are possible from his teachings.
Sichling et al., U.S. Pat. No. 4,379,226, disclosed an optical sensing device which contains a vibrating spring whose frequency of vibration is determined by the parameter p to be measured. Sichling does not specify how fast the parameter p may vary and it is obvious from the embodiments given that high frequency measurements are not possible with his teachings.
Thalman in U.S. Pat. No. 4,591,712 disclosed a sensing apparatus wherein a reciprocal plunger is utilized to alter the amount of light reflected back into an enclosed bundle of fiber optic elements. Thalman does not provide for high frequency capability in his device and it is obvious that his device could not be used for high frequency vibration measurements.
An encapsulated motion transducer has been disclosed in copending application Ser. No. 886,827, filed July 18, 1986, and is designed to operate in hostile environments with a high frequency capability.
The embodiments submitted in the copending application can be used to measure displacement of vibrating objects but there are problems and limitations with those embodiments. The high frequency response of any spring-mass system is limited by the first resonant mode of vibration of the system. A typical response curve for a spring-mass system is shown in FIG. 1. A successful sensor design is one that operates in the flat region below the resonant peak and where the resonant peak is above 10,000 Hz.
The resonant frequency is proportional to the stiffness of the spring and inversely proportional to the mass of the moving elements. Therefor, in the design of a spring-mass system to obtain the highest possible resonant frequency, the designer should strive to achieve the largest spring stiffness and the smallest mass. In the copending application, the mass of the sapphire tip can not be optimized to extremely small values because the the design requires a ball diameter larger than the diameter of the springs. The spring elements are likewise forced to larger than optimum values because the fiber optic elements must pass through the springs in the embodiments shown. The stiffness of the spring elements can not be set at values that are high enough to compensate for the large masses of the embodiments given. High spring stiffnesses cause high contact pressures between the tip and the object surface which can result in contact deformations, permanent denting and other problems. In fabricating and testing embodiments shown in the copending application, the highest resonant frequency that was practically obtainable was approximately 700 Hz. Other problems such as friction among the spring elements and between the tip and casing were found to degrade the performance of devices of the copending application by reducing the actual resonant frequency below the value calculated where frictional effects are not considered.
In the device disclosed by U.S. Pat. No. 4,196,629 to Philips vibration measurements are made up to 10,000 Hz. Thus, there is a continuing need in the state-of-the-art for a contact displacement transducer with high frequency capability to 10,000 Hz.
In U.S. Pat. No. 4,196,629 the outer race of a ball bearing is deflected outward radially in the vicinity surrounding each of the balls, and a fiber optic proximity probe can be used to detect those deflections. Three types of waveforms are disclosed which result from defects on the outer ring, inner ring, or ball. Also explained is the peak to RMS ratio of the waveforms which could be used as an indicator of impending bearing failures.
Experience with bearing failures in rotating machines indicates that defects on bearing component parts often grow to be of quite significant size prior to the initiation of catastrophic failure. For example, cracks or spalls initiated on bearing inner or outer rings have been found to have grown to the point where they cover the entire circumference of the ring. There is therefore, a continuing need in the state of the art of bearing vibration monitoring to be able to determine the size of bearing defects.